Promise.all vs等待Promise.all示例错误

Question

我试图让自己看到Promise.all和await Promise.all之间的不同之处。我了解到，如果其中一个承诺失败，第一个承诺就会提前结束，但在等待的情况下，我们必须等待所有承诺的完成。

在我的示例中，在两种情况下，它们都是同时完成的。我哪里做错了？

​

/**
 * Create a promise
 * @param ok true -> resolve, false -> reject
 */
function create_promise(ok) {
  return new Promise((resolve, reject) => {
    setTimeout(() => ok ? resolve() : reject(), 2e3)
  })
}

// Finish as soon as I get a reject
Promise.all([create_promise(true), create_promise(false), create_promise(true)])
  .catch(() => console.log('rejected'))

// Finish after waiting all
const my_async_function = async () => 
  await Promise.all([create_promise(true), create_promise(false), create_promise(true)])

my_async_function().catch(() => console.log('rejected'))

​

Answer 1

await Promise.all将确保在执行await之后的行之前解析所有承诺。

​

/**
 * Create a promise
 * @param ok true -> resolve, false -> reject
 */
function create_promise(ok) {
  return new Promise((resolve, reject) => {
    setTimeout(() => ok ? resolve() : reject(), 2e3)
  })
}

console.log("async/await version...")
const my_async_function = async () => {
  console.log("before await promise.all");
  const res = await Promise.all([create_promise(true), create_promise(false), create_promise(true)]);
  console.log("after await  promise.all"); // This line will be executed only after all promises are successfully resolved
  return res;
  }
my_async_function().catch(() => console.log('rejected'))

console.log("without async/await version...")
const my_SYNC_function = () => {
  console.log("before promise.all");
  const res = Promise.all([create_promise(true), create_promise(false), create_promise(true)]);
  console.log("after promise.all"); // This line will always be immediately executed after the Promise.all line, and not waiting for the promise completion  at all.
  return res; // So this line returns the return value of Promise.all, which will be an unresolved promise because the 2 seconds have not been elapsed yet.
  }
  
my_SYNC_function().catch(() => console.log('rejected'))

​

这与承诺本身被以某种方式“更快地执行”无关。

然而，调用不带await的Promise.all的函数返回得更早，它只是返回了一些尚未解决的承诺。